Der Spiegel magazine reported that the entire communication system malfunctioned, constituting a serious emergency, saying the crew were forced to plan the landing using an on-board satellite phone.
The delay will complicate Merkel's schedule at the two-day meeting in Argentina, where the Group of 20 industrialised countries already expect to face "very, very difficult" negotiations on myriad issues.
The plane was diverted to the Cologne-Bonn airport and landed without incident. Later, Merkel would say she thought something bad had happened in Germany.
"I'm glad we had the most experienced captain of the German Air Force in command", she said.More news: Too soon to create gene-edited babies, say scientists
In a freaky incident involving Merkel's official plane in 2013, a 24-year-old intruder high on drugs and wearing just underpants managed to get aboard the parked jet in Cologne and stage what media termed "a one-man party" before he was arrested.
Noteworthy, the aircrew set the transponder to code 7600, that means "radio failure".
Later, the delegation travelled by bus to a hotel in Bonn. Later, they boarded another plane to Madrid.
German Chancellor Angela Merkel delivers a speech during the German Ukrainian Economy Forum in Berlin, Germany, Thursday, Nov. 29, 2018.
It was not clear whether planned talks with Presidents Trump, Putin and Xi will be rescheduled.